Funds are requested for a MDS Nordian Inc. Gammacell-40 exactor self- contained low dose rate research animal (rodent) irradiator to support the ongoing NIH-funded individual-initiated research programs in a new state of the art animal facility located within a recently erected research building (BRBII) on the campus of the University of Pennsylvania School of Medicine. Twelve core laboratories comprise the list of the major users of the requested instrument. Each have laboratories in the BRBII and will house their mice and perform ongoing experiments requiring whole animal irradiation and extended maintenance of irradiated animals in ongoing experiments in the animals in the basement of this building. These core laboratories will constitute the majority (>75%) of the usage of the irradiator. In addition, there is a group of over 25 additional users that will utilize this instrument on an infrequent (<25% basis. This latter group is comprised of NIH-funded faculty from University of Pennsylvania School of Medicine associated departments, institutes, and hospitals in addition to other faculty with laboratories in the BRBII. Although two other similar instrument are available on the campus, a specific need for this additional instrument arises because the new animal facility in which the animals of the core user group are housed is a closed facility in which the animals taken to other buildings for irradiation cannot be returned to the investigator's designated area. As a consequence, it is impossible to perform experiments that are either long-term requiring maintaining the animals beyond a single day after irradiation, or require comparison of irradiated and reconstituting models, bone marrow chimeras, induction of immunodeficiency, among others. The University of Pennsylvania School of Medicine has already made a significant financial commitment to support the procurement of this instrument. At a cost of over $33,000, the school has committed funds for an ongoing service contract to maintain and service the instrument, committed funds to compensate an individual to oversee the day to day operation of the instrument, and agree to pay for the moving and setup costs for the irradiator. Scientific oversight will be accomplished by a committee comprised of the PI of the proposal, the scientific director of the Abrahamson Family Cancer Research Institute, and two faculty representatives from the core user group.